Particles respond to an applied acoustic standing wave by transporting to specific locations along the wave (i.e., pressure node, pressure antinode). This relocation is dictated by the contrast factor (i.e. positive contrast, negative contrast) which originates from differences in density and elasticity between the particle and the surrounding media. For example, particles with positive contrast (e.g., incompressible polystyrene beads, cells) in aqueous media are generally transported to acoustic pressure nodes. On the other hand, compressible, silicone elastomeric particles (NACPs) have a negative contrast property that is opposite to commonly used particles (e.g., polystyrene beads)1-2. Consequently, NACPs move to acoustic pressure anti-nodes when subjected to acoustic standing waves, which is a direction opposite from common, incompressible particles.
The capacity to relocate incompressible particles such as cells to pressure nodes has been used in various approaches for focusing and separation of mammalian cells.3-9 For example, the recently commercialized ATTUNE flow cytometer (LIFE TECHNOLOGIES) substitutes traditional hydrodynamic focusing with ultrasonic standing wave fields to focus cells into a single flowing stream prior to laser interrogation.3 To increase the high-throughput capacity of flow cytometry, Piyasena et al. recently developed multi-node acoustic focusing and demonstrated up to 37 parallel flow streams.4 Peterson et al. exploited the inherent contrast factor of constituents from whole blood to separate and sort positive contrast erythrocytes from negative contrast lipids within an acoustofluidic device.5.6 
One current drawback of using negative acoustic contrast elastomeric particles to relocate incompressible particles such as cells to pressure nodes is that the elastomeric particles are not amenable to covalent modification with a specifically desired molecular recognition molecule. For example. PCT Patent Application Publication WO 2010/132474A2 discloses ‘Stable Elastomeric Negative Acoustic Contrast Particles and Their Use in Acoustic Radiation Fields’, but does not teach preparation of stable, elastomeric particles using starting materials with functional groups available for covalent modification with biological moieties. For instance, WO 2010/132474A2 only describes the use of inert silicone starting material (i.e., polydimethyl siloxane (PDMS)) without available groups for biofunctionalization in which to synthesize the elastomeric particles. Recently, Cushing et al. reported using protein adsorption as a way of modifying the surface of such negative acoustic contrast PDMS particles for biomolecule quantification assays.10 While protein adsorption may be convenient, such adsorption techniques often generate heterogeneous surfaces resulting from random orientation and denaturation of proteins on the surface.11 These considerations become more important in cell sorting applications that require high concentrations of active, surface-presenting bioaffinity groups for capturing rare cells and cells with a low quantity of targeted surface antigens.
In addition to the use of negative acoustic contrast elastomeric particles for bioseparations in acoustofluidic devices, negative and positive acoustic contrast particles have utility in many industrial fields such as those fields involving the production of paints, foods, inks, coatings, films, cosmetics, and rheological fluids. Using bulk synthetic approaches to synthesize monodisperse colloids with useful biochemical and mechanical properties represents a longstanding goal in synthetic chemistry, chemical engineering, bioengineering, and mechanical engineering. Rapid and scalable synthesis of vast quantities of monodisperse colloids appeals to many industrial fields involving the production of paints, foods, inks, coatings, films, cosmetics, and rheological fluids.20 Monodisperse colloids also garner significant importance in scientific communities with examples in the production of slurries, clays, minerals, aerosols, foams, macromolecules, sols, semiconductor nanocryistallites, silica colloids, and biochemical interfaces with proteins, viruses, bacteria, and cells.20 
As described above, utilization of acoustic contrast colloids in biological applications, such as diagnostic screenings or immunological bio-marker assays, would require the presence of ample functional groups for various binding and bio-conjugation reactions. The ability to rapidly synthesize functional, monodisperse colloids with controlled mechanical properties (i.e., specific bulk modulus and density) is desirable as it would allow for tight responsive control in acoustic fields. Particles designed with high bulk moduli and densities exhibit positive acoustic contrast coefficients, indicating transport to the acoustic pressure nodes of standing waves.5 Conversely, particles designed with low bulk moduli and densities exhibit negative acoustic contrast coefficients, indicating transport to the acoustic pressure antinodes of standing waves.5 Predicate models for colloid synthesis have failed to fabricate tightly monodisperse colloids with a tunable acoustic response (i.e., exhibiting either positive or negative acoustic contrast by altering the mechanism of synthesis) via bulk synthetic methods.
Accordingly, there remains an unmet need for acoustic contrast particles with functional groups that would allow for a range of binding and bio-conjugation reactions. In addition, there remains an unmet need for monodisperse acoustic contrast colloids that can be produced via bulk synthetic methods, and also for such monodisperse particles that can be produced with covalently modifiable functional groups that can be produced via bulk synthetic methods. The presently disclosed subject matter provides such particles.